Photonic device structure and method of manufacture

ABSTRACT

Disclosed method and apparatus embodiments provide a photonic device with optical isolation from a supporting substrate. A generally rectangular cavity in cross section is provided below an element of the photonic device and the element may be formed from a ledge of the supporting substrate which is over the cavity.

GOVERNMENT RIGHTS

This invention was made with Government support under Agreement NumberHR0011-11-9-0009 awarded by DARPA. The Government has certain rights inthe invention.

FIELD OF THE INVENTION

Structure and method embodiments described herein relate to formingphotonic devices on an integrated circuit substrate with sufficientoptical isolation between a photonic device and the substrate materialto reduce evanescent coupling between them.

BACKGROUND

There is a current trend to integrate photonic devices and electronicdevices on the same semiconductor substrate. A silicon-on-insulator(SOI) substrate can be used as the supporting substrate for suchintegration. When photonic devices such as optical waveguides are formeda cladding is provided around the core of the waveguide for confining alight wave propagated along the waveguide. The core material has anindex of refraction which is larger than that of the cladding. Ifsilicon is used as the core material of a waveguide, having an index ofrefraction of about 3.47, the waveguide cladding can be formed of amaterial having a lower index of refraction. For example, silicondioxide, which has an index of refraction of about 1.54 is often used asthe waveguide cladding.

When a silicon-on-insulator substrate is used as the supportingsubstrate, the cladding material below the waveguide core can be theburied oxide (BOX) insulator of the SOI substrate, which is againtypically silicon dioxide, and the waveguide core can be formed from thesilicon above the BOX insulator. The BOX cladding functions to preventoptical signal leakage by evanescent coupling from the silicon waveguidecore to a supporting silicon substrate of the SOI structure. However, toprevent such evanescent coupling, the BOX cladding material beneath thewaveguide core must be relatively thick, for example, greater than 1.0μm and often 2.0 μm—3.0 μm thick. When the Box cladding material isthick it inhibits heat flow to the underlying silicon, thus diminishingits effectiveness as a heat dissipater, particularly for CMOS circuitswhich may be formed on the same substrate. In addition, when certainelectronic devices, such as high speed logic circuits, are integrated onthe same SOI substrate as photonic devices, the BOX of the SOI substratemust be relatively thin, typically having a thickness in the range of100-200 nm. Such a thin BOX insulator SOI, while providing a goodsubstrate for the electronic devices, is insufficient to preventevanescent coupling of the silicon waveguide core to the underlyingsupporting silicon of the SOI substrate, which causes undesirableoptical signal loss. In addition, SOI substrates are relativelyexpensive and sometimes of limited availability.

Accordingly, non-SOI substrates have also been used to integrateelectronic and photonic devices on the same substrate. One techniquewhich may be used to prevent evanescent coupling of an optical device toan underlying non-SOI substrate is described in U.S. Pat. No. 7,920,770.In this patent, a deep isolation trench is etched in the substrate belowa fabricated optical device. The etching described provides a trenchwhich is formed in a generally curved shape below the optical device. Asnoted, an underlying cladding material for a photonic device, such as awaveguide core, must be at least 1 μm thick, and is preferably 2.0μm-3.0 μm thick. It should also extend at that depth laterally past eachside edge of the photonic device by at least 1 μm. However, to meet thecladding depth criteria a curved trench would require a lateral extentpast the side edges of a photonic device of greater than 1 μm. Thelarger the lateral extent of the curved trench beyond the side edge of aphotonic device, the greater substrate real estate which must beprovided for forming the photonic device. The '770 patent also disclosesthat an additional optical device fabrication material is provided overthe substrate for formation of an optical device.

What is needed is a non-SOI substrate suitable for forming both CMOS andphotonic devices which provides a generally rectangular shaped lowercladding, as well as a simplified method of forming photonic devices andunderlying cladding. A substrate structure which does not require thepresence of an additional optical device fabrication material over anon-SOI substrate is also desired.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates in cross-section one embodiment of a photonic devicefabricated over a substrate;

FIG. 2 illustrates in cross-section another embodiment of a photonicdevice fabricated over a substrate;

FIGS. 3A through 3O illustrate in cross-section a processing sequencewhich can be used to form the FIG. 1 embodiment;

FIGS. 4A and 4A-1 illustrate a cross-section and planar view of aportion of the FIG. 1 embodiment;

FIGS. 4B and 4B-1 illustrate a cross-section and planar view of anotherportion of the FIG. 1 embodiment;

FIGS. 5A through 5F illustrate in cross-section a processing sequencewhich can be used to form the FIG. 2 embodiment; and,

FIG. 6 illustrates a cross-section of a substrate in accordance with theFIG. 1 embodiment which has both electronic devices and photonic devicesfabricated thereon; and

FIG. 7 illustrates a cross-section of a substrate in accordance with theFIG. 2 embodiment which has both electronic devices and photonic devicesfabricated thereon.

DETAILED DESCRIPTION OF THE INVENTION

An embodiment described herein provides a photonic device, e.g., awaveguide which has a core formed from semiconductor substrate material,and an associated lower cladding material provided in a cavity of thesubstrate material. The cavity is located below the photonic device. Anembodiment of a method of forming the photonic device and underlyingcladding is also described.

Embodiments described herein also provide a photonic device, e.g., awaveguide, over a semiconductor substrate in which a lower cladding of agenerally rectangular shape is formed in the substrate.

The various embodiments described herein provide a non-SOI substratewhich is suitable for CMOS and photonic device integration.

FIG. 1 depicts one structural embodiment of the invention in which aportion of a semiconductor substrate 101 has an oxide, e.g., silicondioxide, filled cavity 125 acting as a lower cladding for a photonicdevice. The photonic device shown in FIG. 1 is a waveguide having as anelement thereof a waveguide core 129. The oxide filled cavity 125 has agenerally rectangular shape in cross section. The photonic devicefurther includes an upper cladding in the form of an oxide 135, e.g.,silicon dioxide formed on the sides and over the waveguide core 129. Theoxide 135 formed over waveguide core 129 may be part of an interlayerdielectric (ILD) structure which is formed as part of a CMOS andphotonics circuit interconnect metallization, as described in detailbelow.

The waveguide core 129 is formed from a ledge portion 131 (FIG. 3K) ofthe substrate 101 which is produced during the etch of the substrate 101to produce the generally rectangular shape for oxide filled cavity 125.Since the waveguide core 129 is formed from the same semiconductormaterial in which the oxide filled cavity 125 is formed, no additionalprocessing steps are required to form an additional photonic devicefabrication layer over substrate 101.

FIG. 2 depicts another embodiment in which a substrate 201 is alsofabricated to produce an oxide filled cavity 225 of generallyrectangular shape. The oxide filled cavity 225 acts as a lower claddingfor a waveguide which includes a waveguide core 229 which is providedover an oxide material 203, e.g., silicon dioxide, which acts as asubstrate 201 protection layer. An upper cladding on the sides and overwaveguide core 229 is provided by an oxide 235, e.g., silicon dioxide,formed over waveguide core 229 which may be part of an interlayerdielectric (ILD) structure which is formed as part of a CMOS andphotonic circuit interconnect metallization.

Unlike the FIG. 1 embodiment, the FIG. 2 embodiment has a waveguide core229 formed from a photonics fabrication layer which is provided overoxide layer 225 and an oxide spacer layer 203.

FIGS. 3A through 3O show a processing sequence for forming the FIG. 1structure starting with a semiconductor substrate 101. The substratematerial may be a single crystalline silicon semiconductor material.However, the processing sequence may be used with other substratematerials suitable for CMOS and photonic device fabrication, such aspolycrystalline silicon, silicon carbide, and silicon germanium, amongothers. The processing sequence illustrated in FIGS. 3A through 3O canbe performed before, after or during the processing of substrate 101 toform CMOS devices on another portion of the same substrate 101.

FIG. 3A shows a portion of semiconductor substrate 101 having an uppersurface 104 as the starting point of the process. As shown in FIG. 3B, alayer of a protective material 103 is formed on the top surface ofsubstrate 101. The protective material may be an oxide, for example,silicon dioxide (SiO₂), grown or deposited on the top surface ofsubstrate 101 and protects substrate 101 from subsequent processingsteps. A hard mask material 105, for example, silicon nitride (Si₃N₄) isthen deposited on the protective material 103.

As shown in FIG. 3C, a patterned photoresist material 107 is next formedover the hard mask material 105, the pattern defining an opening 109 inthe photoresist material 107.

As shown in FIG. 3D, the opening 109 is used to etch through the hardmask material 105, protective material 103 and into the substrate 101,forming a substrate trench 111 in an upper surface 104 of substrate 101.The etch to form trench 111 and which etches through the protectivelayer 103 and hard mask 105, can be an anisotropic dry etch. Thephotoresist material 107 is then removed.

FIG. 3E illustrates the formation of a protective liner 113 at the sidesof trench 111, and at sides of protective material 103 and hard maskmaterial 105 at the opening 109. The protective liner 113 may be formedof an oxide, for example, SiO₂ which can be applied by deposition. Sincean area to be protected for subsequent processing is at the sidewalls ofsubstrate trench 111, the protective liner 113, instead of being appliedby deposition, may be grown on trench 111 sidewalls and bottom. Ineither case, and as shown in FIG. 3F, the bottom of protective liner 113in trench 111 is removed by an anisotropic wet or dry etch leaving theprotective liner 113 on the sides of trench 111. If the protective liner113 is applied by deposition, the protective liner 113 is also left onthe sides of opening 109 in the protective material 103 and hard maskmaterial 105. The depth of substrate trench 111, which also determinesthe length 1 (FIG. 3F) of the protective liner 113 along the sides oftrench 111, is selected based on a desired thickness of a subsequentlyproduced ledge portion 131 (FIG. 3J) of substrate 101 used to form anelement of a photonic device as described below. For example, if thedesired thickness of a photonic device element, such as a waveguide core129, has a value in the range of about 30 nm to about 1 μM, then thedepth of trench 111 and consequent length 1 of the protective liner 103along the sides of trench 111 will likewise have the same value in thecorresponding range of about 30 nm to about 1 μm.

After formation of protective liner 113, and as shown in FIG. 3G, anisotropic wet etch of substrate 101 is initiated through opening 109 andtrench 111 which begins to form a cavity 117 in substrate 101. Theetchant will etch the silicon 101 substrate but not the protective liner113 or protective material 103. In order to help minimize the substrate101 area occupied by a photonic device and its associated underlyingcladding, a generally rectangular etch cavity is desired. The manner inwhich a generally rectangular cavity etch of a silicon substrate can beachieved, and appropriate etchants for this purpose, are described inU.S. Pat. Nos. 7,628,932 and 8,159,050 which are incorporated byreference herein in their entirety.

A silicon etch is first performed within trench 111 to open desiredsilicon planes of substrate 101 prior to a wet etch. The wet etchant maybe selected based on desired cavity shape, the shape of desired ledgeportion 131 (FIG. 3J) of substrate 101 and the crystal orientation ofthe substrate. If <100> silicon is used, the wet etchant can be ahydroxide, for example NH₄OH or tetramethylammonium hydroxide (TMAH),though NH₄OH will form a shaped cavity faster than TMAH. The etch ratesof substrate 101 will differ with the direction of the silicon planes.FIG. 3G illustrates the start of the etching in which a generallyhexagonal shaped cavity starts being formed. Further etching produces agenerally diamond shaped cavity 119, as shown in FIG. 3H. Still furtheretching produces the laterally elongated generally hexagonal shapedcavity 121 in FIG. 3I. In order to shape the FIG. 3I cavity into agenerally rectangular shape a different etchant may be used to createsquare corners for the cavity 121. Thus, a buffered fluoride etchsolution having a volumetric ratio of NH₄F:QEII:H₂O₂ of about 4:2:3 maybe used to further isotropically etch the FIG. 3I cavity 121 to formcavity corners which are more square and produce the generallyrectangular shaped cavity 123 shown in FIG. 3J. QEII is a commercialetching solution available from Olin Microelectronics Materials (Newalk,Conn.). The generally rectangular shaped cavity 123 creates a ledgeportion 131 of substrate material over the cavity 123.

Following the wet etch to establish the generally rectangularcross-sectional shaped cavity 123 shown in FIG. 3J, the protectivematerial 103 and hard mask material 105 are removed by dry etch orchemical mechanical polishing to expose the upper surface 104 ofsubstrate 101. The substrate 101 is then washed leaving substrate 101with the generally rectangular cavity 123, ledge portion 131, and aportion of protective liner 113 as shown in FIG. 3K.

As shown in FIG. 3L the cavity 123 is next filled with an oxide material125, for example, silicon dioxide. The oxide material 125 will act as alower cladding for a subsequently formed photonic device and may beformed by deposition, spin coating, or thermal growth.

FIG. 3M shows the formation of a patterned photoresist material 127 overthe substrate 101 ledge portion 131. The photoresist material acts as amask for an anisotropic etch of the substrate 101 and ledge portion 131,which stops at the upper surface of oxide 125 providing an element of aphotonic device in the form of waveguide core 129, FIG. 3N. Thewaveguide core 129 is supported on the upper surface of oxide material125, which provides a lower cladding for waveguide core 129. As shown inFIG. 3O, an upper cladding material in the form of an oxide 135, e.g.,silicon dioxide, is next formed on the sides and over the siliconwaveguide core 129, forming the completed photonic waveguide shown inFIG. 1.

Although the oxide material 125 is described in conjunction with FIG. 3Las formed in the generally rectangular cavity 123 before formation ofthe waveguide core device 129, it is also possible to form the oxidematerial 125 in cavity 123 after formation of waveguide core 129.

FIGS. 4A and 4A-1, depict respective planar and cross-sectional (alongline A-1, A-1) views of a portion of the FIG. 1 structure. The planarview of FIG. 4A shows in dotted lines structures below the oxide 135.The cross-sectional view FIG. 4A-1 is taken through the location of atrench 111 in the substrate 101 through which etching of the generallyrectangular cavity 123 occurs. The oxides 125 and 135 are bothillustrated in the FIG. 4A-1 cross section and oxide 125 is alsoillustrated by dotted lines in the FIG. 4A planar view. As shown in FIG.4A, a plurality of trenches 111 are formed which are spaced in a lineardirection along substrate 101 so a linear extending underlying claddingoxide 125 is provided for a linear extending waveguide core 129.

FIGS. 4B and 4B-1 depict respective planar and cross-sectional (alongline B-1, B-1) views of another portion of the FIG. 1 structure. Theplanar view of FIG. 4B shows in dotted lines structures below oxide 135.The cross-sectional view is taken through a linear extending waveguidecore 129 and shows the waveguide core 129 surrounded by the claddingoxides 125, 135 in the FIG. 4B-1 cross-section and linear extendingoxide 125 and waveguide core 129 are shown by dotted lines in the FIG.4B planar view. The upper cladding oxide 135 can be part of aninterlayer dielectric (ILD) structure used to support a metallizationpattern for CMOS and photonic device connections, in the mannerdescribed below in connection with FIG. 6.

The FIG. 1 embodiment provides an optical device in the form of awaveguide having a core 129 formed of substrate material. The waveguidehas a lower cladding formed in a generally rectangular cavity, as viewedin cross section, of the substrate and provides an easily fabricatedstructure, while reducing the amount of substrate 101 real estaterequired for its formation, as compared to a curved cavity.

FIGS. 5A through 5E depict in cross-section a processing sequence whichmay be used to form the FIG. 2 embodiment.

FIG. 5A illustrates a semiconductor substrate 201 which can be of thesame material as substrate 101, as one example, single crystal silicon.

FIG. 5B illustrates formation of a protective oxide material 203, e.g.,silicon dioxide, over substrate 201. Oxide material 203 can be grown orformed by deposition or spin coating. A photonic device fabricationmaterial 213 is formed over the oxide material 203, for example, bydeposition. Photonic device fabrication material 213 can be singlecrystalline silicon, polycrystalline silicon, amorphous silicon, orother material suitable for forming photonic devices, such as Si₃N₄,Si_(x)N_(y), SiO_(x)N_(y), SiC, Si_(x)Ge_(y), GaAs, AlGaAs, InGaAs, orInP, where x and y are positive integers, (e.g., 1, 2, etc.). Thematerial 213 is selected such that it has a higher index of refractionthan later formed surrounding cladding materials. FIG. 5B alsoillustrates a patterned photoresist 227 provided over the photonicdevice fabrication material 213.

FIG. 5C illustrates the FIG. 5B structure after the photonic devicefabrication material 213 is etched using a web or dry anisotropic etchand using the photoresist 227 as a mask and after removal of thephotoresist 227. An element of a photonic device, e.g., waveguide core229, formed of the photonic device fabrication material 203, remainsover the oxide material 203.

Next, as shown in FIG. 5D, an opening 209 is etched into oxide material203 using a patterned photoresist (not shown). The opening 209 partiallyextends into substrate 201 forming a trench 211 in the substrate 201.Following this, the substrate 201 is further etched using the techniquesand etching materials described above with respect to FIGS. 3G through3K to form a generally rectangular cavity 223 in the substrate whichextends laterally past side edges of waveguide core 229. The etchantwill etch the silicon substrate 201, but not protective material 203.The cavity 223 and opening 209 are then filled with an oxide 225, e.g.,silicon dioxide, formed by growth, deposition or spin coating, as shownin FIG. 5E. As shown in FIG. 5F, an oxide 235 e.g., silicon dioxide, isformed over the waveguide core 229 such that an oxide lower and uppercladding 225, 235 completely surrounds the waveguide core 229. The oxide235 can be separately formed, or can be part of an interlayer dielectric(ILD) used to support a metallization pattern for CMOS and photonicdevice connections, in the manner described below in connection withFIG. 7.

FIG. 6 shows the FIG. 1 embodiments formed on a substrate 101 which hasCMOS circuits formed on one portion of the substrate 101 and a photonicsdevice, e.g., a waveguide, formed on another portion. FIG. 6 also showsthat the upper cladding oxide 135 may be formed as part of an interlayerdielectric (ILD) structure supporting one or more metallization layers141. The CMOS circuits are exemplified by a MOSFET transistor 151 havinga gate 153 with sidewall spacers 163 formed over a gate oxide 155 andhaving source 157 and drain regions 159, and isolated by shallow trenchisolation structures 161. The CMOS devices, e.g., transistor 151, can beformed on substrate 101 before, after or during formation of thewaveguide core 129 and associated oxide filled cavity 125.

FIG. 7 shows the FIG. 2 embodiment formed on a substrate 201 which hasCMOS Circuits formed on one portion of substrate 201 and a photonicdevice, e.g., a waveguide formed on another portion. FIG. 7 also showsthat the upper cladding oxide 135 may be formed as part of an interlayerdielectric (ILD) structure supporting one or more metallization layers141. The CMOS circuits are exemplified by a MOSFET transistor 151 havinga gate 153 with sidewall spacers 163 formed over a gate oxide 155 andhaving source 157 and drain regions 159, and isolated by shallow trenchisolation structures 161. The CMOS devices, e.g., transistor 151, can beformed on substrate 201 before, after or during formation of thewaveguide core 229 and associated oxide filled cavity 225.

As seen in FIGS. 6 and 7, the embodiments illustrated in FIGS. 1 and 2can be part of a common substrate 101 (201) on which both CMOS circuitsand photonic devices and circuits are formed.

While method and apparatus embodiments which are examples of theinvention have been described above, the invention is not limited to thespecifics of those embodiments as changes can be made without departingfrom the spirit or scope of the invention. For example, while awaveguide having an associated core 129 (229) and surrounding upper andlower oxide cladding have been described and illustrated, other photonicdevices can also be constructed using silicon from the substrate 101(FIG. 1), or from the photonics fabricating material (FIG. 2) such as,optical modulators, filters, gratings, taps, light detectors, lightemitters and other photonic devices. Also, transistor 151 is just oneexample of various electronic devices and circuits which can be formedin the CMOS area of substrate 101 (201). Also, although a siliconsubstrate is described in connection with the FIG. 1 embodiment thesubstrate 101 can be formed of other materials suitable for forming anelement of a photonic device. Likewise, the silicon substrate 201described in connection with FIG. 2 may be formed of other materialssuitable for supporting a photonic device.

Accordingly, the invention is not limited by the foregoing description,but is only limited by the scope of the appended claims.

What is claimed as new and desired to be protected by Letters Patent ofthe United States is:
 1. A method of forming a photonic structure,comprising: etching a semiconductor substrate to create a cavity below aledge portion of the semiconductor substrate; and forming an element ofa photonic device from the ledge portion of the semiconductor substrate.2. A method as in claim 1, wherein the photonic device element comprisesa waveguide core.
 3. A method as in claim 1, further comprising: fillingthe cavity with a cladding material.
 4. A method as in claim 3, whereinthe cladding material comprises an oxide.
 5. A method as in claim 4,wherein the semiconductor substrate comprises a silicon substrate.
 6. Amethod as in claim 5, wherein the oxide comprises silicon dioxide.
 7. Amethod as in claim 1, further comprising: using a mask and a first etchto form a trench in a surface of the semiconductor substrate; and, usinga first isotropic etch through the trench to form the cavity.
 8. Amethod as in claim 1, further comprising: forming a hard mask over thesemiconductor substrate; forming a patterned photoresist over the hardmask to define a location for the trench; and etching the semiconductorsubstrate through the hard mask to form the trench.
 9. A method as inclaim 8, further comprising forming an oxide material between the hardmask and semiconductor substrate.
 10. A method as in claim 7, furthercomprising: forming an etch protective liner on sidewalls of the trenchbefore performing the first isotropic etch.
 11. A method as in claim 10,wherein the etch protective liner comprises an oxide.
 12. A method as inclaim 11, wherein the oxide comprises silicon dioxide.
 13. A method asin claim 7, wherein the first isotropic etch comprises a hydroxide etch.14. A method as in claim 13, wherein the hydroxide etch comprises usingNH₄OH or TMAH as the etchant.
 15. A method as in claim 13, furthercomprising performing a second isotropic etch following the firstisotropic etch to etch the cavity.
 16. A method as in claim 15, whereinthe second isotropic etch comprises using a buffered fluoride as theetchant.
 17. A method as in claim 16, wherein the buffered fluorideetchant solution comprises NH₄F:QEII:H₂O₂.
 18. A method as in claim 15,wherein the forming of the photonic device element occurs after thesecond isotropic etch.
 19. A method as in claim 3, wherein the cavity isfilled with the cladding material after the photonic device element isformed.
 20. A method as in claim 3, wherein the cavity is filled withthe cladding material before the photonic device element is formed. 21.A method as in claim 2, further comprising forming a cladding materialover and around the sides of the photonic device element.
 22. A methodas in claim 21, wherein the cladding material over and around the sidesof the photonic device element comprises an oxide.
 23. A method as inclaim 22, wherein the oxide comprises silicon dioxide.
 24. A method asin claim 1, wherein the cavity has a generally rectangularcross-sectional shape.
 25. A method of forming a photonic structure,comprising: forming a photonic device element above a semiconductorsubstrate; and etching a generally rectangular shaped cavity in thesemiconductor substrate below the photonic device element.
 26. A methodas in claim 25, wherein the photonic device element is formed from aledge portion of the semiconductor substrate residing above the cavity.27. A method as in claim 25, wherein the photonic device element isformed from a photonic fabrication material provided above the cavity.28. A method as in claim 25, wherein the generally rectangular shapedcavity is formed by at least a first isotropic etch of the semiconductorsubstrate using a first isotropic etch solution and a second isotropicetch of the semiconductor substrate using a second isotropic etchsolution.
 29. A method as in claim 28, wherein a first isotropic etchsolution comprises a hydroxide etch solution and the second isotropicetch solution comprises a buffered fluoride etch solution.
 30. Aphotonic structure comprising: a substrate comprising a substratematerial suitable for forming a photonic device element; a photonicdevice element comprising the substrate material; and a cavity in thesubstrate below the photonic device element and containing a claddingmaterial for optically decoupling the photonic device element from thesubstrate.
 31. A photonic structure as in claim 30, wherein thesubstrate material comprises a semiconductor material.
 32. A photonicstructure as in claim 31, wherein the semiconductor material comprisessilicon.
 33. A photonic structure as in claim 32, wherein the siliconcomprises single crystal silicon.
 34. A photonic structure as in claim30, wherein the cladding material comprises an oxide.
 35. A photonicstructure as in claim 34, wherein the oxide comprises silicon dioxide.36. A photonic structure as in claim 30, further comprising an uppercladding material over the photonic device element.
 37. A photonicstructure as in claim 36, wherein the upper cladding material comprisesan oxide.
 38. A photonic structure as in claim 37, wherein the oxidecomprises silicon dioxide.
 39. A photonic structure as in claim 36,wherein the upper cladding material comprises a part of an interlayerdielectric (ILD) of a metallization structure.
 40. A photonic structureas in claim 30, wherein the photonic device element is a waveguide core.41. A photonic structure as in claim 30, wherein the cavity has agenerally rectangular shape.
 42. A photonic structure comprising: asubstrate having a generally rectangular cavity in cross section andcontaining a material for optically decoupling a photonic element fromthe substrate; and the photonic element formed above the cavity.
 43. Astructure as in claim 41, wherein the photonic device element is formedfrom a fabrication material provided over the substrate.
 44. A structureas in claim 41, wherein the photonic device element is formed from partof the substrate provided over the cavity.